Ignition coils for internal combustion engines adapted to be directly coupled to spark plugs are known. For example, see U.S. Pat. Nos. 5,706,792 and 5,736,917, the disclosures of which are hereby incorporated herein by reference. It is known that conventional ignition coils (or pencil coils) utilize both a primary coil and a secondary coil. The primary coil is typically a low voltage coil, while higher voltages are induced in the secondary coil.
Prior art FIG. 2 illustrates a conventional spool 100 upon which a secondary coil is wrapped or wound. Secondary spool 100 includes a cylindrical mid-section 102 upon which the windings 106 are wrapped. Spool 100 further includes planar flanges 104 extending outwardly at approximately a 90 degree angle from each end of the mid-section.
Unfortunately, the provision of this type of flange 104 at both ends of the spool is problematic. For example, the 90 degree angle or corner 108 defined by the flange and the end of the mid-section wind-on surface may cause a voltage concentration between the secondary and primary windings, at the high voltage end of the secondary winding.
Another problem inherent with flanges 104 at both ends of the spool arises upon thermal expansion. The thermal coefficient of expansion of the spool is typically much higher than that of the wire 106. There may be relative movement between the flange 104 and the end 110 of the wire segment upon exposure to high temperatures, thereby creating a gap adjacent one or both of flanges 104. When a flexible encapsulant is used, this gap could allow a collapse of the winding, which could increase stress when the coil returns to a hot condition. The collapse could also increase the wire to wire voltage gradient which may lead to electrical failure.
It is apparent from the above that there exists a need in the art for an ignition coil or pencil coil: having a secondary spool requiring a lessor number of wire turns/wraps; allowing for a reduction of voltage concentration between the secondary winding and the primary spool; and/or for minimizing adverse effects upon separation between the spool and coil upon thermal expansion/contraction.